Incandescent bulbs are commonly used in a variety of applications to provide light. For example, incandescent bulbs may be used as a light source for illuminated switches, lighted panels, displays, legends, indicators, and in a variety of other applications.
Although incandescent bulbs may provide a satisfactory degree of illumination, they also carry with them a number of disadvantages. For example, incandescent bulbs operate at a relatively high temperature. Consequently, incandescent bulbs can generate enough heat to cause burns when used in some applications, such as in lighted switch or panel applications. In addition, incandescent bulbs have a relatively short life span, and may require frequent replacement. Likewise, many incandescent bulbs are prone to failure in high vibration environments. Finally, incandescent bulbs operate at relatively high power levels.
Light emitting diodes (LEDs) offer advantages over incandescent bulbs in each of the above areas. Thus, when compared to incandescent bulbs, LEDs produce less heat, operate for a longer life, are less prone to failure in high vibration environments, and consume less power. Because of these advantages, it is desirable to substitute LEDs for incandescent bulbs in many applications.
Unfortunately, LEDs produce a different luminance, or brightness level, than incandescent bulbs given the same input current or voltage. FIG. 1 illustrates the relative luminance of an incandescent bulb and an LED given a varying input voltage. The LED luminance curve is indicated by the reference numeral 10 while the incandescent bulb luminance curve is indicated by the reference numeral 12. As FIG. 1 illustrates, LEDs and incandescent bulbs may have quite different luminance levels over a wide range of input voltages.
Similarly, FIG. 2 illustrates the relative luminance of an incandescent bulb and an LED over a varying input current. The LED luminance is indicated by the reference numeral 14, while the incandescent bulb luminance is indicated by the reference numeral 16. As FIG. 2 illustrates, LEDs and incandescent bulbs may have quite dissimilar luminance levels depending upon the input current level.
While there are many uses in which it is desirable to replace an incandescent bulb with an LED of similar luminance, one application of particular importance is in aircraft cockpits. For many aircraft, display and indicator lights must be designed in accordance with specifications for brightness. In addition, under certain conditions, the aircraft pilot may wish to manually dim the display by adjusting a dimmer switch. If each of the lights has similar brightness characteristics, the display may be dimmed consistently. This is particularly important when the pilot is wearing night vision goggles. At such times, the pilot must be able to darken the display entirely. If any of the display lights may not be darkened, the night vision goggles may "bloom," rendering them practically useless. Accordingly, in many applications LEDs may only be substituted for incandescent bulbs if the brightness characteristics are the same.
The present invention is directed to providing a compensation circuit for matching the luminance of an LED to that of an incandescent bulb over a wide range of input currents or input voltages.